Optical communications systems typically include a pair of network nodes connected by an optical waveguide (i.e., optical fiber) link. Within each network node, communications signals are converted into electrical signals for signal regeneration and/or routing, and converted into optical signals for transmission through an optical link to another node. The optical link between the network nodes is typically made up of multiple concatenated optical components, including one or more optical fiber spans interconnected by optical amplifiers.
The use of concatenated optical components within a link enables improved signal reach, that is, the distance that an optical signal can be conveyed before being reconverted into electrical form for regeneration. For example, optical signals are progressively attenuated during propagation through a span, and amplified by an optical amplifier (e.g., an erbium doped fiber amplifier (EDFA)) prior to being launched into the next span; however, signal degradation due to noise and dispersion effects increases as the signal propagates through the fiber. Consequently, noise and dispersion degradation are significant factors in limiting the maximum possible signal reach.
Chromatic dispersion, also known as group velocity dispersion, in a single mode fiber is a result of two mechanisms: (1) waveguide dispersion wherein different wavelengths of light propagate in the fiber at different speeds; and (2) material dispersion wherein the phase velocity of plane waves in glass varies with wavelength. Hereinafter, references to “chromatic dispersion” are understood to mean the sum total of group velocity dispersion effects.
Mathematically, first order chromatic dispersion is the derivative of the time delay of the optical path with respect to wavelength. The effect of chromatic dispersion is measured in picoseconds of arrival time spread per nanometer of line width per kilometer of length (ps nm−1 km−1). The magnitudes of waveguide dispersion and material dispersion vary with wavelength, and at some wavelengths the two effects act in opposite senses. The amount of chromatic dispersion present in a link can also vary with the temperature of the fiber and any change in the communication path introduced by optical switching. Chromatic dispersion in an optical fiber presents a serious problem when using a light source having a non-ideal spectrum, for example, a broad or multi-line light source, or when high data rates are required (e.g., over 2 GB/s).
Polarization mode dispersion (PMD), also known as differential group delay, is a result of imperfections in the optical fiber that lead to different propagation speeds for orthogonal polarization components of an optical signal. The imperfections can be due to geometric asymmetry of the fiber core and material birefringence. Both effects can arise from manufacturing processes and from thermal and mechanical stresses present in the field. Moreover, the magnitude of polarization mode dispersion can vary rapidly in time (e.g., at rates that exceed 10 KHz).
Chromatic dispersion is proportional to the square of the baud rate of an optical signal while polarization mode dispersion is linearly proportional to the baud rate. Consequently, chromatic dispersion is the limiting factor for high baud rate (e.g., greater than 10 Gbaud) communication systems for lengths exceeding a few kilometers.
Various modulation formats and techniques for receiver and transmitter equalization to mitigate the effects of chromatic dispersion and polarization mode dispersion are known in the art. For example, multi-level intensity modulation with direct detection (IM-DD) using four-level amplitude shift keying (ASK-4) has been used to achieve the desired dispersion tolerance. In effect the baud rate is reduced by a factor of two, leading to an improvement in dispersion tolerance by a factor of four; however, the multi-level modulation results in a noise penalty of at least 5 dB compared to a non-return-to-zero (NRZ) signal at twice the baud rate. Consequently, the reach of the system is reduced by almost a factor of four. Differential quadrature phase shift keying (DQPSK) can be used to achieve a dispersion tolerance and noise tolerance similar to direct detection at half the baud rate; however, the additional expense to implement a DQPSK format makes it less cost-effective.
Receiver equalization techniques for improved direct detection performance are known. These techniques include maximum likelihood sequence estimation (MLSE) equalization, maximum a posteriori (MAP) equalization and turbo encoding/decoding. For example, a receiver for 10 Gbaud direct detection using MLSE-5 can compensate for chromatic dispersion in up to 400 km of optical fiber; however, to increase the MLSE-5 by an additional state approximately doubles the gate count of the equalizer to obtain an additional 50 km of chromatic dispersion tolerance. At rates greater than 10 Gbaud, the MLSE equalizer alone is insufficient to compensate for chromatic dispersion. For example, an MLSE-5 equalizer for 40 Gbaud only compensates for up to approximately 20 km of chromatic dispersion. Quadrupling the gate count for an MLSE-7 receiver only incrementally increases the chromatic dispersion compensation to approximately 28 km as an MLSE equalizer alone does not scale with baud rate to compensate for chromatic dispersion. This lack of scalability for chromatic dispersion compensation with baud rate similarly applies to other decoders such as those used for MAP equalization and turbo decoding.
Other techniques such as a chirped pulse technique described in U.S. Pat. No. 4,979,234 titled “Saturated Semiconductor Laser Amplifier for Compensation of Optical Fibre Dispersion,” for managing chromatic dispersion in optical systems are known; however, these techniques also do not scale well with increasing baud rate. For example, application of the chirped pulse technique can achieve a doubling of the dispersion tolerance so that the dispersion tolerance for a 40 Gbps signal improves from about 3.6 km to about 7.2 km; however, this improvement is an insignificant change for longer reach communication systems.
For the purposes of analyzing the effects of chromatic dispersion and polarization mode dispersion, it is convenient to represent an optical communications system using the block diagram of FIG. 1. In this case, the transmitter is represented by an electrical-to-optical converter (E/O) 4 which operates to convert an electrical input signal x(t) into a corresponding optical signal XOPT(ω)) for transmission to a receiver. The optical fiber span 8, including all concatenated components, is represented by a transfer function H1(ω))H2(ω)), where the components are normally complex, H1(ω)) represents the contribution due to chromatic dispersion and H2(ω)) represents the contribution due to polarization mode dispersion. The receiver is represented by an optical-to-electrical converter (O/E) 12 which detects the instantaneous power of optical signal YOPT(Ω)) received through the optical fiber span 8, and generates a corresponding electrical output signal y(t).
In general, the output signal y(t) represents a distorted version of the input data signal x(t). While it is highly desirable for H1(ω))H2(ω)) to be approximately one, this is rarely the case. Accordingly, it is frequently necessary to utilize at least some form of compensation, so that the original input data signal x(t) can be detected within the distorted output signal y(t).
One common method of addressing signal degradation due to chromatic dispersion in high-bandwidth communications systems is to insert one or more optical dispersion compensators 16, represented in FIG. 2 by the compensation function C(ω)), to compensate for chromatic dispersion caused by the remainder of the link. Since chromatic dispersion is largely insensitive to polarization and varies little over time, compensators based on bulk dispersion compensation fiber often provide satisfactory performance. Some compensators also provide a time variable amount of compensation which enables mitigation of time-variant dispersion effects. In either case, the compensators are intended to at least partially offset the signal distortions. The compensation function C(ω)) is a dispersive function that is selected to optimize performance of the link for chromatic dispersion but in general does not address degradation due to polarization mode dispersion introduced by the link. The compensation function C(ω)) is preferably equivalent to the complex conjugate of the chromatic dispersion transfer function H1(ω)) in which case H1(ω)) C(ω))=1. If polarization mode dispersion were not present, the combined effect of H1(ω)) and C(ω)) would be an undistorted output signal YOPT(ω)) that exactly corresponds to the original optical signal XOPT(ω)). Limitations of optical components and the time-varying amount of compensation required make this objective difficult to achieve. Additionally, the compensators represent an additional cost and introduce significant optical losses. These losses are offset by means of additional optical gain which introduces more optical noise. The additional (or higher-performance) optical amplifiers required to provide this increased gain further increase the total cost of the communications system. In addition, the presence of compensators for chromatic dispersion and high performance amplifiers distributed along the length of the link provides a significant technical barrier to system evolution. For example, implementation of optical switching (e.g. at the transmitter and/or receiver end of the link, or at an intermediate site without electrical termination) necessarily requires adjustment of optical amplifiers in order to accommodate changing energy states within the link.
U.S. Pat. No. 7,382,984 titled “Electrical Domain Compensation of Optical Dispersion in an Optical Communications System,” incorporated herein by reference, describes a method to compensate for chromatic dispersion in an optical communications system. According to the method and with reference to FIG. 3, a communications signal x(t) is modulated in the electrical domain according to a chromatic dispersion compensation function C1(ω)). The predistorted electrical signal x1(t) is used to modulate an optical source to generate a corresponding optical signal X1OPT(ω)) for transmission through the optical fiber span 8. In effect, the E-field of the optical signal X1OPT(ω)) is controlled according to the predistorted electrical signal x1(t). Thus the optical signal Y1OPT(ω)) present at the receiver 12 has little or no chromatic dispersion; however, polarization mode dispersion typically remains. In a complementary manner, an optical communications system can utilize a chromatic dispersion function in the electrical domain at the receiver; however, this represents a limited capability for direct detection modulation systems as the compensation function is nonlinear and the phase information is not available.
Compensation for polarization mode dispersion requires the tracking of variations in the polarization components of the optical signal X1OPT(ω)) and the differential delay for these components. To compensate for polarization mode dispersion at the transmitter, access to both polarization components is required, resulting in a significant additional cost to the communications system. Moreover, the polarization states and the polarization mode dispersion H2(ω)) introduced by the optical fiber span 8 can vary rapidly over time. For example, the frequency of the variations in polarization mode dispersion can exceed 10 KHz. The transmitter requires knowledge of the polarization states and differential delay; however, this information is only available at the receiver. Although the receiver can send this information to the transmitter, the latency in reporting the information to the transmitter can make compensation of polarization mode dispersion at the transmitter impractical. More specifically, by the time the information is received at the transmitter, the polarization mode dispersion imparted by the optical link may have changed so that the information is no longer useful.
The present invention addresses the problems set forth above and provides a convenient and cost-effective technique for mitigating the effects of chromatic dispersion and polarization mode dispersion on high bandwidth optical signals.